1. Field of the Invention
This invention relates to a magnetic disc driving device for rotationally driving a disc on which servo signals are recorded in an integer number of sectors formed by dividing each track on the disc along the track direction. More particularly, it relates to a magnetic disc driving device for controlling the position relative to the disc of a magnetic head recording and/or reproducing information signals on or from the disc, using the servo signals recorded on the disc.
2. Description of the Related Art
With a sector servo system magnetic disc in which servo signals are recorded in sectors formed by dividing the track on the disc by an integer number and which is driven in accordance with a servo control system, a sector servo pattern SSP is arrayed at a leading part of each sector. Thus the sector servo patterns and data regions are alternately arrayed on the magnetic disc.
With a magnetic disc having a diameter of 2.5 inches, as shown in FIG. 1 , the entire disc surface is divided into 71 sectors, with each sector being 241.5 .mu.s long. With a magnetic disc having a diameter of 3.5 inches, the magnetic disc surface is divided into 90 sectors, with each sector being 181.5 .mu.s long. The servo pattern SSP is recorded in a leading region of each sector within a range of 18.625 .mu.s from the leading end position.
A typical example of the sector servo pattern is explained by referring to a schematic view of FIG. 2. The sector servo pattern is the pattern as proposed by the present Applicant in JP Patent Application No. 3-32084.
That is, in the sector servo pattern shown in FIG. 2, predetermined reference signals, such as pre-burst signals BS, reference signals AGC for signal level compensation and synchronizing signals SYNC, for example, are recorded in the wake of a write/read (W/R) settling region. Among the Servo pattern signals, the synchronizing signals SYNC include servo header data SH, clocks CLK and pattern sync data PAT SYNC.
In addition, first burst signals X, each recorded with a recording track width T.sub.BW so that its track center TC is coincident with even-numbered tracks 2, 4, 6, . . . , and second burst signals Y, each recorded with a recording track width T.sub.BW so that its track center TC is coincident with odd-numbered tracks 1, 3, 5, . . . . Besides, first address data AD0, AD1, AD2, . . . are arrayed for every two tracks so that their values are changed in the vicinity of the track centers TC of the even-numbered tracks as sector pattern data. Similarly, second address data DO, D1, D2, . . . are arrayed for every two tracks so that their values are changed in the vicinity of the track centers TC of the odd-numbered tracks as sector pattern data.
The magnetic disc driving device detects the address desired to be found from the results of comparison of the first level detection signals X and the second level detection signals Y and compensates the detected address by the second address data to detect the magnetic head position relative to the magnetic disc.
Furthermore, tracking control burst servo signals A and B of the sector servo pattern signals are recorded on the magnetic disc in a checkerboard pattern so that the signals A and B are arrayed at right angles to the tracks and in a staggered relation to each other as shown. Finally, third burst servo signals C as the predetermined reference signals are recorded at a predetermined frequency. The burst signals A, B and C make up fine pattern signals FP. The sector servo pattern on the magnetic disc is formed by pre-recording all of the above-mentioned signals.
The header data of the magnetic disc is a header having a width T.sub.W as shown in FIG. 2 and is employed for servo control by scanning the sector servo pattern for reading tracking control magnetic head position data.
The burst signals A and B making up the fine pattern signals FP are recorded with an offset of one-half the width T.sub.BW of the track on the magnetic head relative to the track center of each track and additionally with an offset relative to each other. The signal detected via a magnetic head when the burst signals A and B of the fine pattern signals FP are scanned by the magnetic head has its envelope changed depending on the scanning position of the magnetic head. It is by detecting changes in the envelope signals that the magnetic head driving device detects the deviation of the magnetic head position. In distinction from the servo signals A and B, the third burst signal C, as the fine pattern signal, is recorded for outputting an envelope level which remains perpetually constant in the radial direction. The third burst signal C is used for normalizing the signal level. For finding the position signals of the magnetic head relative to the magnetic disc, the burst signals A, B and C, pre-recorded on the magnetic disc, are read by scanning by the magnetic head for deriving signal levels V.sub.A, V.sub.B and V.sub.C from the fine pattern signals A, B and C, and by processing in accordance with an equation EQU (V.sub.A -V.sub.B)/V.sub.C
In each sector of the magnetic disc, resulting from division of each track into an integer number of sectors, a data region for recording data is arrayed in the wake of the servo pattern region in which there are recorded the aforementioned servo pattern data. Meanwhile, preambles or postambles are occasionally provided in a track of the magnetic disc. The integer number herein means an integer number N of sectors formed by dividing the track region excluding a preamble region and a postamble region.
In general, the magnetic disc driving device for rotationally driving the magnetic disc comprises a spindle motor, a magnetic head at the distal end of a head arm as a part of an actuator, a read/write circuit, a head position detection unit, a memory (RAM), a digital signal processor (DSP), a voice coil motor (VCM) driving circuit and a voice coil motor.
The magnetic disc driving device employs, as position information data for tracking controlling of the magnetic head, the above-mentioned fine pattern data A, B of the sector servo pattern via the magnetic head. The fine pattern data A, B are formed by burst signals which are detected by the magnetic head to find signal levels V.sub.A, V.sub.B which in turn are employed as position signals. Meanwhile, the magnetic head position in the magnetic disc driving device is known to be proportional to the difference of two signal levels (V.sub.A -V.sub.B). By taking advantage of this relationship of proportionality, the magnetic disc driving device generates position information signals indicating the position of the magnetic head relative to the magnetic disc.
However, should there be servo pattern data dropout in the fine pattern region within the servo pattern, the position control voltage of the magnetic head, as found from the above-mentioned level difference (V.sub.A -V.sub.B), becomes different from the control voltage which is used for moving the magnetic head to a true position. The result is that, with the magnetic head driving device, the control voltage value in case of a data dropout in the fine pattern region becomes different from the voltage value corresponding to the true control position of the magnetic head in case of the absence of data dropout in the fine pattern region. Such voltage difference represents an error in the magnetic head movement control and tends to produce disturbances in the movement control of the magnetic head. The result is that, in the worst of the cases, the magnetic disc driving device causes the magnetic head to be deviated off the track being scanned.